1. Field of the Invention
The present invention relates to a method for forming micro patterns of a semiconductor device, and more particularly to the illumination method and apparatus used to form such micro patterns.
2. Description of the Prior Art
The recent trend to fabricate highly integrated semiconductor devices has been greatly affected by the development of techniques of forming patterns having a micro dimension. Photoresist film patterns formed by a photolithography process are widely used as masks for carrying out an etch process or ion implantation process in the fabrication of semiconductor devices.
A method for reducing the wavelength of light emitted from a light source is known to improve the resolution of a stepper.
For example, steppers using an i-line laser having a wavelength of 365 nm as a light source have a resolution capable of forming patterns having line/space dimensions of about 0.5 .mu.m. Steppers using the KrF laser having a wavelength of 248 nm or the ArF laser having a wavelength of 193 nm as a light source have a resolution capable of forminq patterns having line/space dimensions of about 0.25 .mu.m.
In order to form micro patterns, a method has been proposed which uses a phase shift mask utilizing an optical interference between neighboring patterns. A polarized beam illumination method also exists and uses a polarized beam from the light source of a stepper.
Where a photoresist film pattern with a desired structure is formed on a wafer using the lithography process, illumination is carried out using a mask having a desired light shielding pattern, for example, a reticle.
In this connection, a conventional illumination method used in the formation of micro patterns will now be described in conjunction with FIG. 1.
FIG. 1 is a schematic view illustrating the construction of a stepper used to carry out a general illumination method. The illumination method is the Kohler illumination method.
As shown in FIG. 1, the stepper includes a light source 1. A light beam 2 emitted from the light source 1 passes through an aperture 3 and a condensing lens system 4. The light beam 2 emerging from the condensing lens system 4 then passes through an illumination mask 5 provided with a light shielding pattern 5a, thereby forming an image. The light beam 2 then passes through a projection lens system 6 and reaches a wafer 7.
In accordance with the illumination method wherein illumination is carried out through the above-mentioned optical path, the time taken for illumination is short. However, a problem in that a reduction in yield occurs because the depth4 of focus is small.
In order to solve this problem, a modified illumination method has also been proposed which uses an aperture having a modified shape in such a manner that a light shielding portion is provided at the central portion of the aperture, thereby allowing light to be slantly incident on an illumination mask. In accordance with this method, two light beams diffracted by the illumination mask, namely, the 0th diffracted light beam and the +1'st or -1'st diffracted light beam are condensed on a projection lens disposed beneath the illumination mask. When the 0th and .+-.1'st diffracted light beams have the same incident angle, the depth of focus may be increased.
In accordance with this method, the light shielding portion of the aperture serves to shield light beams incident on the central portion of the aperture. As a result, light reaching the illumination mask and wafer exhibits a low intensity as compared to that in the above-mentioned general illumination method. For this reason, it is necessary to perform the illumination for a lengthened period in accordance with the modified illumination method, as compared to the general illumination method. This results in a degradation in yield.
In order to solve this problem, various methods have been proposed. For example, there is a linearly polarized beam illumination method wherein light emitted from a light source is linearly polarized in a desired direction. In accordance with this method, it is possible to form an image having a line/space width of up to 0.175 .mu.m using an illumination mask having a light shielding film pattern parallel to the polarization direction even when a stepper using an i-line light source is used. However, where an illumination mask having a light shielding film pattern perpendicular to the polarization is used, a degradation in the contrast of the image occurs. In other words, a contrast gap exists.
In cases using the conventional illumination utilizing linearly polarized beams, off-axis illumination or phase shift mask, a high contrast is exhibited when the main axis of linearly polarized beams are parallel to the light shielding film pattern of the illumination mask (S.sub.com components of incident light at an angle of polarization .chi. corresponding to 90.degree.), as compared to the case wherein the main axis of linearly polarized beams are parallel to the light shielding film pattern of the illumination mask (P.sub.com components of incident light at an angle or polarization .chi. corresponding to 0.degree.). As a result, a contrast gap exists between the above-mentioned two components of the incident light. Thus, such a contract gap exists in all cases using the conventional illumination, off-axis illumination and phase shift mask.
The contrast gap increases as the partial interference degree .sigma., namely, the ratio between the transmission area of the aperture and the entire area of the aperture and the feature size decrease. The contrast gap also increases as the numerical aperture increases. Such a contrast gap also increases when the off-axis illumination or phase shift mask is used.
Since a large contrast gap exists when using the conventional illumination method for forming micro patterns, it is difficult to form micro patterns of illumination masks oriented in various directions using the linearly polarized beam illumination method. As a result, it is difficult to achieve a high integration of semiconductor devices.
For this reason, the conventional illumination methods for forming micro patterns involve a reduced process margin, thereby decreasing process yield and reliability.